Slot cathode



1967 B. P. DE LANY ETAL 6 SLOT CATHODE 2 Sheets-Sheet 1 Filed March 4,1963 .M r my a W W? M g m a FZJA/ wa w ZY United States Patent Ofihce3,304,456 Patented Feb. 14, 1967 3,304,456 SLUT CATHODE Beatrice PearsonDe Larry, Miami Beach, Fla, and Paul L. Copeland, Bensenville, TIL;Gertrude I. (Iopeland, executor of the estate of said Paul L. Copeland,deceased, assignor of one-half interest to Gertrude P. Copeland,individuality Filled Mar. 4, 1963, Ser. No. 262,446 19 Claims. (til.313-482) This invention relates to an electrical discharge tube. It hasfor one object to provide in an electrical discharge tube, a cathode ofincreased efficiency, whereby large currents may be established at lowvoltages.

Another object is to provide a construction by which a moderatepotential difference will produce a current in the glow discharge from asmall cathode which is of the order of those usually obtained in an arc.

A still further object is to provide a hollow cathode and to shape thatcathode to make more rapid and certain the initiation of the dischargeWithin the cathode and the transition from the exterior surface of thecathode.

Another object is to form a cold cathode lighting tube operable atconventional voltages.

Other objects will appear from time to time throughout the specificationand claims.

The invention is illustrated more or less diagrammatically in theaccompanying drawings wherein:

FIGURE 1 is a sectional view taken through one form of the device,

FIGURE 2 is a transverse sectional detail of the cathode of FIGURE 1, onan enlarged scale,

FIGURE 3 is a version similar to FIGURE 2, illustrating a modifiedconstruction of the cathode slot edge,

FIGURE 4 is a side elevation of a further modified form of electrode,

FIGURE 5 is a side elevation of a still further modification of thecathode,

FIGURE 6 is an enlarged side elevation, in part section, of a furtherform of cathode, 4

FIGURE 7 is a graph of voltage vs. current for a cold cathode tube of aconventional type, and

FIGURE 8 is a graph of voltage vs. current comparing a conventional tubeand a tube formed in accordance with the present invention.

Like parts are designated by like characters throughout thespecification and drawings.

It should be understood that the present invention finds utility as acold cathode lighting tube of the fluorescent type. In such case therewould be two similarly formed electrodes, the polarity of which wouldreverse each half cycle with the applied alternating current. However,for purposes of illustration, one electrode will be called the anode andthe other the cathode.

As shown in the form of FIGURE 1, the device includes an envelope 1,which is preferably formed of glass but may be formed of anyair-impervious electrically insulating material. 2 is an anodepositioned within the envelope and connected to a lead-in wire 3, bymean of which the anode may be put into electrical connection within asuitable system. The anode 2 may be formed of a variety of metals.Tungsten is one suitable metal.

Within the envelope is positioned a cathode 4, which is secured to alead-in wire 5 which passes through and extends outside of the envelopewall, as shown at 6. The lead-in wire 6 is connected in circuit to asuitable electrical system. Thus, the wires 3 and 6, when the device isto be used, are connected to a source of electrical current.

The cathode 4 may be formed of a wide variety of metals. Refractorymetals may be used to permit the temperature of the cathode to rise.Among suitable metals for the formation of the cathode are tungsten,tantalum, molybdenum, nickel and stainless steel. The cathode is notlimited to any particular metal. The cathode may also be made of ceriumor other rare earths.

As shown in FIGURES 1, 2 and 3, the cathode 4 is generally tubular. Itsone end may be closed, for example, by a member 7, to which the lead-inwire 5 is secured. The end of the cathode may be closed by means otherthan the added plate or plug member 7. That member serves only as oneconvenient closure for the tube 4- and is a convenient attachment meansfor the leadin Wire 5.

The cathode 4 is open at it inner end, as at 8, and is provided alongits side with a slot 9. This slot may run substantially from end to endof the tube 4. It need not do so, however, but it must be of substantiallength. It may be formed merely by slotting all or a part of the lengthof the tube. As shown in FIGURES 1 and 2, the slot 9 is formed merely bya saw or comparable tool which leaves relatively thick edges 10, 10along the slot R.

The slot in the tubular electrode 4 may be formed to produce relativelysharp edges 11, 11 along the slot 9, as shown in the modified form ofFIGURE 3.

The modified form of the cathode shown in FIGURE 4 is generally tubularand closed at both ends. Thus the tubular member 12 is closed at itsouter end by member 13, to which is secured the lead-in wire 5. At itsinner end, the cathode tube 12 may be closed by a member 14. Adjacent tothe inner end of the tubular cathode 12 is an opening 15. This is ofrelatively large diameter and is formed in the side wall of the cathode,adjacent its inner end. Although the hole or opening is shown as beinground, in fact it may be of almost any shape.

Formed in the tubular cathode I2 is a slot 16, generally similar to theslot 9 shown in FIGURES 1, 2 and 3. It communicates with the opening 15.Actually, it need not communicate with that opening as long as one endof the slot 16 approaches closely to the opening 15. The slot Id neednot extend the full length of the tubular cathode 12, but it should beof substantial length and should be in communication with, or closelyapproach, the opening or perforation 15.

A still further modification of the cathode is shown in FIGURE 5. Asshown there, the cathode comprises a generally tubular member 17, closedat its outer end by the parts 18, to which is secured the lead-in wire5. The tubular cathode 17 is open at its inner end, as at 19, and isprovided with a generally helical slot 20, which at its upper endcommunicates with the open end 159 of the cathode 17.

When the expression outer end of the cathode has been used, it is usedto indicate the end of the cathode which is remote from the anode.correspondingly, when the expression inner end of the cathode has beenused, it refers to the end of the cathode which is more nearly adjacentthe anode.

FIGURE 6 illustrates still another form of cathode. A hollow body 21 isclosed at its outer end 22 and includes openings or perforations 23which may be random or arranged and which extend generally the length ofthe cathode. The body 21 may be formed of perforated or expanded metalor formed in any other way to permit positive ions to enter the insideof the cathode through substantially its entire length to causesecondary emission of electrons from within the cathode.

Several forms of the tubular cathode have been shown. In each, there isat, or adjacent, the inner end of the cathode, a relatively largeopening. In three forms of the cathode, there is an extended and narrowslot, or slit, or opening in the wall of the cathode. This relativelynarrow opening communicates at one end with the larger opening, orclosely approaches that opening. Thus, the slots 9 and are incommunication with the open end of the tubular cathodes 4 and 17respectively. In the case of the cathode 12, when both ends are closed,the large opening adjacent the inner end is in communication with, orclosely approached by, the extended slot-like opening 16. Although thecurved and generally helical slot 20, shown in FIGURE 5, is a suitableform for the extended narrow opening, it is shown also as an indicationthat the slot or narrow opening need not be straight, as the slot 9 isstraight, and need not be axially aligned with the tubular cathode, asthe slots 9 and 16 are. The relatively narrow slot-like opening in thecathode wall may be disposed in a variety of positions as long as it isof sufiicient length and closely approaches or communicates with themajor opening, such as the open end 8 or the opening 15. In the fourthform the cathode is formed with many openings or perforations. What isimportant in all forms is to permit positive ions from the exterior toenter the cathode and cause secondary emission of electrons from theinterior cathode surface.

The use, operation and function of the invention are as follows:

FIGURES 7 and 8 are graphs illustrating the effect of providing a meansfor allowing positive ions to enter the cathode cavity generallythroughout its length. In FIG- URE 7, a tube with a -inch diametercathode and containing argon gas was kept at a temperature of -78.6centigrade and at a gas pressure of 280 microns. The curve at the leftside of FIGURE 7 shows an increase in current with an increase involtage. The increase in current is fairly linear. After the voltagereached a level of roughly 800 volts the curve was discontinuous andthere was a substantial increase in current. In fact the voltage wasreduced considerably at this point, but the current still remained at avery high level.

In a tube of this type which does not have any means, except the mainorifice, for permitting positive ions to reach the inside of the cathodehole, the glow is on the external surface of the cathode and does notpenetrate the cavity or hole. Conditions are such that a plasma is notestablished within the cavity. A continuing increase in voltage willultimately establish a plasma within the cavity at which point therewill be a sudden and substantial increase in current. This is shown inFIGURE 7. Once the plasma is established within the cavity, the tubeprovides excellent voltage regulation and the voltage will staygenerally almost constant for increases in current. The action of thehollow cathode is so effective that the parts of the cathode outside ofthe cavity may not have any discharge. It is necessary and essential toestablish a plasma within the cavity so that positive ions will strikethe inner wall of the cavity and provide secondary emission ofelectrons. It is in this way that there can be a rapid increase in thecurrent provided by the tube.

The curves of FIGURE 8 show the effect of a slotted cathode. The valuesfor the curve at the left side of FIGURE 8 were taken from a tube with acathode having a -inch outer diameter and a -inch inner diameter. Theargon gas was at a pressure of 340 microns. The curve at the right oralong the bottom of FIGURE 8 was with an identical cathode except that a;-inch wide slot was milled along one side of the cathode. This slotopened into the cathode cavity. Note that in the left-hand curve ofFIGURE 8 there were small increases in current for rather substantialincreases in voltage. No plasma was formed within the cavity. In theslotted cathode however there was an immediate plasma formed within thecavity and hence very large currents were obtained at 250 volts. Notethat there was excellent voltage regulation. By providing a slot in theside of the cathode or some other means permitting the positive ions tomove into the cathode cavity and strike its wall it is possible to havesubstantial secondary electron emission from the wall. The secondaryemission electrons stabilize the glow within the cathode and permitsubstantially higher currents at conventional voltages. Very largechanges of current can be produced by rather small changes of voltage.This characteristic is accounted for by the rapid variation of secondaryemission with primary energy.

The action of a hollow cathode depends on trapping electrons in theplasma and providing positive ion bombardment of the interior cathodesurface to produce secondary electrons. The slot provides means forpositive ions to enter the central plasma and provides means forcontrolling the rate of loss of positive ions from the central plasma tothe space outside of the hollow cathode.

While it has been stated that the cathode may be formed of a variety ofdifferent metals, the metal of which the cathode is made should be onehaving a high melting point and a low work function. Also, alkali earthsand rare earths may be used if present in a very thin film on asupporting surface of other material of higher melting point. Generally,although alkali earths may be used, they are among the less satisfactorypossible metals for the formation of the cathode. Uranium zirconium carbide is a satisfactory material.

While it is convenient to form the cathode as a tube of constantdiameter, this is not essential and the cathode tube need not be ofuniform diameter from end to end of the tube.

Envelope 1, when the device is assembled, will have the air exhaustedfrom it and a suitable gas under pres sure will be introduced into andretained within the envelope under pressure. The operation of the deviceof the present invention depends on the ionization of this gas. Theactual pressure chosen depends generally upon the size of the cathodestructure. The permissible range of pressure of gases present in theenvelope is from 5-100 mm. of Hg. Experience has shown that the slottedcathode of this invention can operate both at higher and lower gaspressures than can an unslotted cathode. Thus, the use of the slottedcathode of this invention increases the operating range of thepermissible gas present.

The invention is not limited in this use to a particular gas. The noblegases are among those which may be used. Mercury vapor or others may beused.

What is claimed is:

1. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being slit and formed with acavity therein facing in the direction of the anode.

2. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being slit and formed with acavity therein facing in the direction of the anode, and said envelopecontaining inert gas under pres sure of from 5-100 mm. of mercury.

3. In combination in an electric discharge tube an airimperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being slit and formed with acavity therein facing in the direction of the anode, the length of saidcavity being of the order of twenty (20) times its diameter.

4. In combination in an electric discharge tube an airimperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode, a lead-in wire in electrical connection with saidcathode and extending to a point outside of the envelope, said cathodebeing longitudinally slit and formed with a cavity therein facing thedirection of the anode.

5. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode, a lead-in wire in electrical connection with saidcathode and extending to a point outside of the envelope, said cathodebeing longitudinally slit and formed with a cavity therein facing in thedirection of the anode, said slit being approximately inch in diameter,and said envelope containing inert gas under pressure of from 5-400 mm.of mercury, the length of said cavity being of the order of twenty (20)times its diameter.

6. In combination in an electric discharge tube an airimperviousenvelope of insulating mate-rial, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode having an openend facing said anode, a lead-in wire in electrical connection with saidcathode and extending to a point outside of the envelope, said cathodebeing slit and formed with a cavity therein facing in the direction ofthe anode.

I. In combination in an electric discharge tube an air-imperviousenvelope of insulation material, an anode extending thereinto, a hollowmetallic cathode having an open end facing said anode, a lead-in wire inelectrical connection with said cathode and extending to a point outsideof the envelope, said cathode being slit and formed with a cavitytherein facing in the direction of the anode, and said envelopecontaining inert gas under pressure of from 5-100 mm. of mercury.

8. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode having an open end facing said anode, a lead-in wire inelectrical connection with said cathode and extending to a point outsideof the envelope, said cathode being slit and formed with a cavitytherein facing in the direction of the anode, the length of said cavitybeing of the order of twenty (20) times its diameter.

9. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode having an openend facing said anode, a lead-in wire in electrical connection with saidcathode and extending to a point outside of the envelope, said cathodebeing longitudinally slit and formed with a cavity therein facing in thedirection of the anode.

It). In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode having an open end facing said anode, a lead-in wire inelectrical connection with said cathode and extending to a point outsideof the envelope, said cathode being longitudinally slit and formed witha cavity therein facing in the direction of the anode, said slit beingapproximately inch in diameter, and said envelope containing inert gasunder pressure of from 5-100 mm. of mercury, the length of said cavitybeing of the order of twenty (20) times its diameter.

11. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being longitudinally provided witha narrow slit opening into its interior and formed with an extendedopening therein facing in the direction of the anode.

12. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode, a lead-in wire in electrical connection with saidcathode and extending to a point outside of the envelope, said cathodebeing longitudinally provided with a narrow slit opening into itsinterior and formed with an extended opening therein facing in thedirection of the anode, and length of said cavity being of the order oftwenty (20) times its diameter.

13. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being longitudinally provided witha narrow slit opening into its interior and formed with an extendedopening therein facing in the direction of the anode, said slit being incommunication with said extended opening.

14. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowmetallic cathode longitudinally spaced from said anode, a lead-in wirein electrical connection with said cathode and extending to a pointoutside of the envelope, said cathode being longitudinally provided witha narrow slit opening into its interior and formed with an extendedopening therein facing in the direction of the anode, said slit being incommunication with said extended opening, and length of said cavitybeing of the order of twenty times its diameter.

15. In a cold cathode gas filled lighting tube, an anode at one end ofthe tube and a cathode at the other end, said cathode being somewhatcylindrical and generally hollow, and means extending through a majorportion of the length of the cathode for permitting positive ions fromthe exterior of the cathode to enter the generally hollow portion of thecathode and to cause secondary emission of electrons from the interiorof the cathode.

16. The structure of claim 15 further characterized in that said cathodeis formed of a perforated metal.

17. In combination in an electric discharge tube an air-imperviousenvelope of insulating material, an anode extending thereinto, a hollowcathode of refractory metal alloy, at lead-in wire in electricalconnection with said cathode and extending to a point outside of theenvelope, said cathode being slit and formed with a cavity thereinfacing the anode.

18. In combination in an electric discharge tube an airimperviousenvelope of insulating material, an anode extending thereinto, a hollowcathode of a conducting compound, a lead-in wire in electricalconnection with said cathode and extending to a point outside of theenvelope, said cathode being slit and formed with a cavity facing in thedirection of the anode.

19. The method of producing a hollow cathode with perforationsdistributed at random over the entire surface of the enclosing shell bythe process of sintering in a suitable form a coarse powder ofrefractory material under conditions of temperature and pressure suchthat irregularly spaced holes between the particles are left in thecathode structure, which holes provide numerous paths for the injectionof positive ions into the cathode cavity.

(References on following page) References Cited by the Examiner UNITEDSTATES PATENTS Skellett 313-192 X Seibt 313189 Lubcke 313-348 X Krieger313186 X Butler 313341 Depew 3133 Depp 313185 Townsend 313185 Story313-184 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

1. IN COMBINATION IN AN ELECTRIC DISCHARGE TUBE AN AIR-IMPERVIOUSENVELOPE OF INSULATING MATERIAL, AN ANODE EXTENDING THEREINTO, A HOLLOWMETALLIC CATHODE LONGITUDINALLY SPACED FROM SAID ANODE, A LEAD-IN WIREIN ELECTRICAL CONNECTION WITH SAID CATHODE AND EXTENDING TO A POINTOUTSIDE OF THE ENVELOPE, SAID CATHODE BEING SLIT AND FORMED WITH ACAVITY THEREIN FACING IN THE DIRECTION OF THE ANODE.